Along with growing environmental awareness, control on lead that is remarked as being harmful to human bodies has been launched. In Europe, ELV directive (End-of Life Vehicles directive) for limiting use of lead in the vehicle, and RoHS (Restriction of the use of certain Hazardous Substances in electrical and electronic equipment) directive for inhibiting use of lead in electrical and electronic equipment have been put into effect. Generally, solder as the junction material used for electrically coupling parts for electrical and electronic equipment contains lead. The solder is classified into three types, that is, high-temperature solder, intermediate-temperature solder and low-temperature solder in terms of melting point. The intermediate-temperature solder has been developed and put into practical use as Sn—Ag—Cu-base solder and Sn—Cu-base solder. The low-temperature solder has also been developed and put into practical use as Sn—Bi-base solder, and Sn—In-base solder. Those types of solder have been adapted to comply with the ELV and RoHS directives. Meanwhile, the high-temperature solder as high leaded solder contains lead by 85 wt. % or more. The lead-free material as the one alternate to the high leaded solder has not been developed yet, which has been out of the control of ELV and RoHS directives. The high leaded solder contains lead by 85 wt. % or more which may place burden on the environment greater than the burden placed by the Sn—Pb eutectic solder inhibited under RoHS directive. Development of the substitute material alternate to the high leaded solder has been highly demanded.
FIG. 1 illustrates an example of applying the high heat-resistant connection. FIG. 1 is a sectional view showing structure of a semiconductor apparatus. FIG. 2 is a sectional view for explaining flash resulting from the remelted solder.
Referring to FIG. 1, a semiconductor device 1 is connected onto a frame 2 (die bonding) via a solder (junction material). Inner lead of a lead wire 5 and an electrode of the semiconductor device 1 are wire bonded via a wire 4, which then will be sealed with a sealing resin 6 or inactive gas for manufacturing a semiconductor apparatus 20.
The semiconductor apparatus 20 is subjected to a reflow soldering to a printed board via an Sn—Ag—Cu-based intermediate lead-free solder. The melting point of the Sn—Ag—Cu-base lead-free solder is high at approximately 220° C. In reflow connection, the high leaded solder with melting point equal to or higher than 290° C. is employed for die bonding the semiconductor device 1 so as to prevent remelting of the die bonded portion.
The melting point of the currently developed intermediate lead-free solder such as Sn—Ag—Cu-base solder is approximately 220° C. When it is used for die bonding the semiconductor device 1, the solder will be melted upon re-flow connection of the semiconductor apparatus 20 to the printed board. If the area around the connected portion is molded with resin, when the inner solder is brought into molten state, the volume is expanded to cause flashing as shown in FIG. 2, which is the phenomenon that causes the solder 3 to leak from the interface between the sealing resin 6 and the frame 2. Even if it does not leak, it at least acts to escape. As a result, large void 7 may be generated in the solidified solder, resulting in a defective product. As the prospective alternative material, Au-base solder such as solders of Au—Sn-base, Au—Si-base, Au—Ge-base, Zn-base solder such as Zn-base, Zn—Al-base, and Bi-base solder such as Bi, Bi—Cu, Bi—Ag have been reported.
The Au-base solder contains Au by 80 wt. % or more, which has difficulty in realizing versatility in terms of costs. Additionally, the resultant solder is hard but brittle. The Bi-base solder is hard but brittle, and has the heat conductivity of approximately 9 W/m·K which is lower than that of the existing high-temperature solder. It is therefore difficult to apply the above-described solder to the power semiconductor apparatus and power module which require high radiation performance. The Zn-base solder such as Zn and Zn—Al exhibits high heat conductivity of approximately 100 W/m·K, but is easily oxidized, which may prevent sufficient connection in the atmosphere with high oxygen concentration. As it is relatively hard alloy, there may be the risk of cracking the semiconductor device upon connection.
“Patent Document 1” discloses the connection material which overcomes disadvantages of the Zn—Al-base solder which is hard and unlikely to be wet. In the disclosed method, Zn line, Al line and Zn line are sequentially laminated using the clad material formed by clad rolling. With this method, wettability is ensured by the Zn-base layer on the surface, and stress buffer capability is provided by the soft Al-base layer as the inner layer to ensure junction reliability. Each melting point of Zn and Al is 420° C. and 660° C., respectively. The melting point of Zn—Al eutectic (Zn-6Al) produced through reaction between Zn and Al is 382° C. The junction material with high melting point provides high heat-resistant property.